1. Field of the Invention
The present invention relates to a multi-layer anti-reflection coating to be applied to a substrate and more particularly, to an anti-reflection coating design that is particularly susceptible to production line manufacturing.
2. Description of the Prior Art
A considerable number of anti-reflection coatings have been suggested in the prior art for a primary design purpose of ensuring that the residual reflectance will be held to a relatively small value over the entire range of the visual spectrum. While experience has shown that single or double layer coatings have provided significant improvement, the residual reflectance is still less than desired, and the range of suppressing the reflectance over the visual spectrum is necessarily limited.
To improve these restrictions, the prior art has resorted to anti-reflection coatings having three or more layers, such as disclosed in U.S. Pat. No. 3,604,784. This three-layer anti-reflection coating includes an outer or first layer adjacent the air medium of MgF.sub.2 (N=1.38) with an optical thickness of .lambda.o/4 wherein .lambda.o is a predetermined design wavelength approximately in the center of the visual spectrum. The second layer is a mixture of oxides of titanium and Al.sub.2 O.sub.3 (N.congruent.2.00) with an optical thickness of .lambda.o/2. The third layer adjacent the glass substrate is of Al.sub.2 O.sub.3 or MgO (N=1.64 to 1.72) with an optical thickness of .lambda.o/2. A major limitation for this type of multi-layer anti-reflection coating is that it generally has sufficient anti-reflection effect only on a glass substrate with a refractive index in the range of 1.68 to 1.88. Regrettably, this necessitates an expensive glass composition.
The prior art has further attempted to obtain anti-reflection coatings that would be effective for all the conventional glass substrates, and a variety of four-layer anti-reflection coatings have been proposed. These coatings include the structure disclosed in U.S. Pat. No. 3,781,090 which has the following construction of layers, in sequence from the air side to the glass substrate side, the first layer is made of a material having a low index of refraction (N=1.35 to 1.62); the second layer has a high index of refraction (N=2.00 to 2.30); the third layer has a medium index of refraction (N=1.56 to 1.72), while the final fourth layer has a low index of refraction (N=1.35 to 1.62). This design provides a four layer anti-reflection coating with enhanced anti-reflection effects on a glass substrate having a refractive index in the range of 1.42 to 1.72 as a result of adjusting each layer thickness in accordance with the refractive index of the glass substrate. While an improvement in the anti-reflectance characteristics of the coating is found, there are still a number of disadvantages in the structure disclosed in this patent. In controlling the optical thickness of each deposited layer, an optical monitoring method using light interference has been generally utilized. In implementing this method, glass of low refractive index, such as soda lime glass (N.congruent.1.52), is often used as a monitor because of its low cost and easy attainability. For enhancing the accuracy of the thickness control, it has been found desirable that the fourth layer, which is the first coated on the monitor glass, is made of a material whose refractive index is significantly different from that of the monitored glass. However, the design of U.S. Pat. No. 3,781,090 utilizes a fourth layer that is composed of a material having a low index of refraction in the range of 1.35 to 1.62 which is only slightly different from that of the monitored glass. Thus, accurate control of the optical thickness of the fourth layer is particularly difficult with the optical monitoring method. Finally, it is considered desirable, for enhancing the optical characteristics, that MgF.sub.2, which has the greatest difference in refractive index from the monitored glass from the practical applicable materials available in anti-reflection coatings, be used for the fourth layer. It is also desirable that MgF.sub.2 be used as the first or outer layer since it resists scratching and has one of the lowest indices of refraction of all the practical applicable materials for coating. A problem exists, however, when using a layer of MgF.sub.2, since when deposited, it is microscopically uneven across its surface as compared to other materials. Thus, by incorporating MgF.sub.2 into two layers in a four layer anti-reflection coating, the resulting surface of the coating is considerably uneven. This results in a decrease in durability as demonstrated by a standard adhesion test which is performed by abrading the coating with a stream of fine silicon carbide particles dropped from a known height.
U.S. Pat. No. 3,892,490 is cited of general interest for disclosing a monitor system for controlling the depositing of layers upon a glass substrate. U.S. Pat. Nos. 3,565,509, 3,463,574, 3,432,225, and 3,235,397 are cited of general interest.